In general, aluminum alloy heat exchangers are manufactured by brazing. A paste brazing material, a brazing material foil, a brazing sheet and the like are used in brazing. In particular, when joining an extrusion tube and a fin, clad fin (that is, a brazing sheet) is generally used.
A brazing sheet is a clad material comprising an aluminum alloy plate as a core, one side or both sides of the core being applied with a brazing material. With respect to the brazing sheet used for the heat exchanger, 4000 series aluminum alloy is applied as the brazing material on one side or both sides of the core.
For tubes made by sheet forming, the brazing sheet can be used, and a mono-layer material or a clad material can be selected for tubes, inner fins, and outer fins.
However, when an extrusion tube, particularly an extrusion material having complicated cross-sectional shape, is intended to clad, the metal cannot be uniformly clad during extrusion, thereby forming a mixture of different alloys at some portions. Therefore, extrusion tubes with simple shapes can only be clad. For example, clad materials can be manufactured for round tubes. However, tubes with complicated cross-sectional shapes such as oblate shapes that are used for heat exchangers, cannot be clad. That is, when a heat exchanger using an extrusion tube is manufactured, fins are generally used as the clad material when brazing is conducted, since the extrusion tubes cannot be clad.
A method for brazing extrusion tubes without using the brazing sheet has been suggested. For example, Patent Document 1 discloses a brazing method using a binder and an aluminum alloy powder for brazing. However, the specification merely mentions the viewpoint of joining, and there is no description regarding the corrosion resistance, which is an important issue in the heat exchanger.